1. Field of the Invention
This invention generally relates to downhole well tools and specifically to shaped charge perforating guns for subterranean wells.
2. Description of Related Art
Traditional petroleum drilling and production technology often includes procedures for perforating the wall of a production well bore to enhance a flow of formation fluid along perforation channels into the fluid bearing strata. Depending on the well completion equipment and method, it is necessary for such perforations to pierce the casing, production pipe or tube wall. In many cases, the casing or tube is secured to the formation structure by a cement sheath. In these cases, the cement sheath must be pierced by the perforation channel as well.
There are three basic methods presently available to the industry for perforating wells. Those three methods are: a) explosive propelled projectiles, b) pressurized chemicals and c) shaped charge explosives. Generally, however, most wells are perforated with shaped charge explosives.
Shaped charge explosives are typically prepared for well perforation by securing a multiplicity of shaped charge units within the wall of a heavy wall, steel pipe joint. The pipe joint bearing the shaped charges may be supported at the end of a wireline, coiled tube, coupled pipe or drill string for location within the wellbore adjacent to the formation zone to be perforated by detonation of the shaped charges.
Collectively, a pipe joint and the associated charge units will be characterized herein as a “charge carrier.” One or more operatively coupled charge carriers providing a single operating unit of extended length shall be characterized herein as a “perforating gun.” A perforation gun is merely one of many “bottom-hole assemblies” or bottom-hole tools the present invention is relevant to.
Each shaped charge unit in a charge carrier comprises a relatively small quantity of high energy explosive. Traditionally, this charge unit is formed about an axis of revolution within a heavy steel case. One axial end of the shaped charge unit is concavely configured. The concave end-face of the charge is usually clad with a thin metallic liner. When detonated, the explosive energy of the decomposing charge is focused upon the metallic liner. The resulting pressure on the liner compressively transforms it into a high speed jet stream of liner material that ejects from the case substantially along the charge axis of revolution. This jet stream penetrates the well casing, the cement sheath and into the production formation.
A multiplicity of charge units is usually distributed along the length of each charge carrier. Typically, the shaped charge units are oriented within the charge carrier to discharge along an axis that is radial of the carrier longitudinal axis. The distribution pattern of shaped charge units along the charge carrier length for a vertical well completion is typically helical. However, horizontal well completions may require a narrowly oriented perforation plane wherein all shaped charge units in a carrier discharge in substantially the same direction such as straight up, straight down or along some specific lateral plane in between. In these cases, selected sections of charge carriers that collectively comprise a perforation gun may be joined by swivel joints that permit individual rotation of a respective section about the longitudinal axis. Additionally, each charge carrier is asymmetrically weighted to gravity bias the predetermined rotational alignment when the gun system is horizontally positioned.
In situ petroleum, including gas and oil (crude oil), is often found as a gaseous or viscous fluid that substantially saturates the interstices of a porous geologic strata. In some cases the petroleum bearing strata is distributed over an expansive area having a relatively small thickness. For example, a porous strata saturated with crude oil may extend for miles in several directions at a nominal depth of about 6500 ft. but with only a 10 to 20 ft. thickness. A normal or vertical penetration of the strata to extract the crude could only have about 10 ft. of perforated production face. Notwithstanding an abundant total of petroleum reserves present in the strata (formation), the production rate through one well would be relatively small. To efficiently drain the formation, numerous such wells would be required. The enormous cost of each well is well known to the industry.
In cases as described above, the producer may elect to amplify the fluid production from a single well by increasing the length of the well production face within the fluid bearing formation. Generally, such production face increases are achieved by guiding the well borehole direction along a plane located at or near the bottom of the formation and substantially parallel with the lay of the formation. Such a completion strategy has been characterized in the art as Extended Reach Drilling (ERD). Using ERD, the producer may penetrate the formation with a production face length of 6,000 ft., for example. Typically, however, 6,000 ft. of substantially horizontal, perforated well production face along a geologic formation that is 6,500 ft. beneath the earth's surface may require a total, deviated borehole length that is as much as 35,000 ft. (7 miles).
Following prior art technologies, a mile of horizontal well bore is usually perforated in increments: each requiring a separate round trip. There are several factors contributing to such relatively short perforation length increments in ERD completions. Most factors, however, relate to the length and, hence, weight, of perforating gun structure that may be positioned in the wellbore adjacent to the fluid production zone. One such factor, for example, is the structural or mechanical strength capacity of the support string (wireline, tubing, drill string or derrick) to support the suspended weight of a full length perforating gun that is constructed predominately of steel. In the case of the above example, a full length gun may be 5,000 to 6,000 feet long. At a representative weight distribution rate of 14.75 #/ft. for example, such a gun would weigh 75,000 to 90,000 lbs.
Another factor that limits the length of a traditional perforating gun that is assembled with a plurality of heavy steel charge carriers according to prior art practice, is the magnitude of axially imposed “push” force along the perforation gun axis necessary to overcome the friction force bearing on the perforating gun surface as it is pressed by gravity against the bottom elements of the wellbore wall.
That portion of a wireline, drill string or coiled tubing suspended vertically below the drilling platform is supported entirely by the casing head or by the derrick structure. As the course of the wellbore direction departs from vertical and becomes increasingly horizontal, the wellbore direction enters an angular zone of repose. The “angle of repose”, usually measured relative to the horizontal plane, is that angle from horizontal at which static frictional forces acting on a structure at the supporting surface interface are greater than the gravity forces (potential energy) on the same structure. In brief restatement, the angle of repose is the maximum surface slope that will statically sustain the position of a structure on the surface. If the surface slope angle is increased above the angle of repose, static friction force on the structure is exceeded by gravitational force and the structure begins to slide downwardly along the surface. The term “angle of repose” and associated concept is to be distinguished from the term and concept associated with “deviation angle” which is a wellbore direction angle measured from vertical.
Coiled tubing, coupled tubing or pipe, and drill pipe are bottom-hole assembly support strings that have some compressive force transfer capacity. Wirelines have little or no capacity to transmit compressive force but nevertheless support considerable weight in the tensile mode. The mass of a tubing or pipe support string in a borehole above the angle of repose transfers a pushing force to that portion of a support string below the angle of repose. At some point, however, the frictional force on the support string below the angle of repose exceeds the compressive force from the support string above the angle of repose. Typically, the coefficient of friction between a pipe or coiled tubing string and a wellbore wall may be about 0.50 lb drag/lb normal wt. At that point of force equilibrium, natural forces will position the bottom-hole assembly no deeper along the wellbore. To increase borehole penetration of the bottom-hole assembly, external force must be applied.
Responsive to a need for external force to push a bottom-hole assembly further along a horizontal borehole, the prior art has engaged a mobility tool often characterized as a “tractor.” The tractor is a mechanical device driven by a hydraulic circulation stream within a pipe or tubing suspension string or by an electric motor served by a wireline supported electrical conduit. The device is positioned in the support string above the bottom-hole tool assembly/perforating gun. Driving surfaces on the tractor, such as wheels having a serrated perimeter or circulating tracks with lugs, engage the borehole wall and “push” the heavy steel perforating gun along the wellbore wall. At the present state of development, tractors may be capable of 4,500 to 5,000 lbs. thrust.
A typical 5 in. perforating gun assembled from heavy steel charge carriers may have an air environment weight of about 14.75 #/ft. Nominally, steel has a specific gravity of about 7.83. When immersed in water having a density of about 62 #/ft3 as is often found in a downhole environment, the weight distribution of the perforating gun is reduced by about 8.45 #/ft. Buoyancy of a structure is a function of the volume of fluid displaced by the structure and the weight of that displaced volume.
For an atypical example, assume a 5 in. perforating gun having a 0.1363 ft3/ft. volumetric displacement envelope. The gun has an air weight distribution of about 14.75 #/ft. and a downhole weight distribution in water of about 6.30 #/ft. This gun is to be pushed by a tractor along a 6000 ft. horizontal completion bore that imposes a coefficient of friction of 0.5 # drag/# normal weight along the gun length. The tractor in the suspension string is assumed to have a maximum thrust of about 4,500 lb. A generalized approximation of the maximum gun length that may be positioned in the horizontal wellbore may be determined as follows:    [0.5 lb drag/lb nor.wt.(coeff. of friction)]×6.30 # wt./ft. gun=3.15 # drag/ft. gun    [4,500 lb thrust(tractor)]/3.15 # drag/ft. gun=1429 ft. gun
Accordingly, the perforation operation is limited to a maximum gun length of 1429 ft. Therefore, 4 to 5 round trips into the well are required to shoot the full length of the 6,000 ft. perforation zone. However, only the first shot may be under underbalanced pressure conditions. More will be subsequently explained about underbalanced pressure conditions.
Proposals have been made to supplement the tractor technology with strategically placed carriage wheels along the perforating gun to reduce the coefficient of friction element of the equation. If effective as proposed, distributed carriage wheels may decrease the overall coefficient of friction by half or more. Consequently, only 2 to 3 round trips to complete the well perforation of 6000 ft. would be required. At the same time, however, the addition of wheels to the gun structure reduces the useful gun diameter and increases the gun weight. Furthermore, several shaped charges and respective production perforations may be sacrificed for each carriage wheel on the gun. Most damaging, however, is the loss of useful gun diameter which has the consequence of reducing the maximum size of shaped charge unit that may be used in the gun and hence, the size and depth of perforation.
Although tractor technology provides means to increase the length of a horizontal perforating gun, such means remain insufficient to position a single, 6000 ft. perforating gun of unified length in a substantially horizontal wellbore. Such completions are still burdened by the need for incremental perforation procedures and multiple “round trips” into the well.
There is a standing desire of all deep well producers to complete the well in as few trips as possible: preferably only one. Rig time on a well location is measured in thousands of dollars per hour. The rig time required for a 35,000 foot round trip may be several, 24 hour days. This is not borehole advancement time (drilling) but merely the task of withdrawing a bottom-hole tool or assembly, whether drill bit or perforating gun, and returning with another. Obviously, 4 or 5 round trips into and out of a 35,000 foot well is enormously expensive.
The expense of multiple trips to complete a horizontal production bore is not the only penalty of a multiple trip completion. Petroleum bearing earth strata are not often of uniform porosity and/or permeability. A flow conducive pressure differential of greater in situ pressure in the formation than in the wellbore is characterized as an underbalance. Degrees of minimum underbalance necessary to extract full flow from a particular area of production zone may be highly variable along the borehole length. Also highly variable is the minimum underbalance necessary to flush the perforation channel of perforation debris. To clean up the perforations and start the flow of formation fluid into the wellbore along the perforation channels in one area of a formation may require an underbalance of only 500 psi pressure differential between the formation pressure and the wellbore pressure. Along another area of the same formation, a 2,000 psi differential of underbalance may be required to initiate flow and clean up the perforations.
The well producer is afforded only one opportunity to perforate an underbalanced well at the pressure differential required by the formation circumstances. At the time of that one opportunity, the well pressure may be drawn down to or near the greatest pressure differential required to induce flow from the most reticent flow area. Following the first gun shot, it is no longer possible to reduce the internal wellbore pressure significantly below the in situ formation pressure. Consequently, any subsequent shot increments necessary to complete a multiple gun perforation must be made at a substantially balanced well pressure. Accordingly, many of the flow reticent perforation channels may not be flushed of perforation debris and therefore fail to produce the fluid flow rate that may otherwise be expected.
Both long and short length horizontal completions may be plagued by a reduction of shaped charge penetration capacity. Predominately, a horizontal wellbore is perforated upwardly to induce a gravity expulsion of debris from the perforation channels. However, prior art perforating guns generally rest against the floor of the horizontal wellbore when the shot is taken. Due to the fact that the wellbore diameter is significantly greater than the perforating gun diameter, the shaped charge perforation jets must leap the asymmetry gap before effective perforation begins. Traversal of the asymmetry gap consumes and diverts a significant portion of the jet energy thereby reducing the penetration capacity. In a perfect world, the uppermost surface element of the perforation gun would be positioned in contact juxtaposition with the uppermost surface elements of the wellbore at the moment of an upwardly directed shaped charge ignition.